A known transmission apparatus for an automobile or the like is disclosed in JP2000-110923A (paragraph 0022, FIGS. 1, 3 and 7), which contains therein a transmission gear mechanism and a differential mechanism that serves as a final driving mechanism. The transmission apparatus, which basically has the structure illustrated in FIGS. 5-8, includes a transmission gear mechanism 120, a differential mechanism 125, and a casing 110. The transmission gear mechanism 120 is provided with a pinion (i.e., speed-reducing small gear) 122a, of which diameter is small, at an axial one side of a shaft, an axial one side which serves as an output side (upper side as viewed in FIG. 6). The differential mechanism 125 is provided with a ring gear (i.e., speed-reducing large gear) 126 at an axial one side (lower side as viewed in FIG. 6) of a shaft which is far away from a differential gear. The ring gear 126 is always gear-meshed with the pinion 122a. The casing 110 includes, therein, a first chamber S101 in which the transmission gear mechanism 120 is housed, and a second chamber S102 in which the differential mechanism 125 is housed. The casing 110 of this conventional transmission apparatus is mainly configured with a first case 111 and a second case 115, both of which are separated members and are fixedly connectable with each other in a liquid-tight manner. The first case 111 includes a first inner surface 110a facing, at a short distance away, an end surface of the ring gear 126 positioned at an axial outer side (lower side as viewed in FIG. 6). The second case 115 includes a second inner surface 110b facing, at a short distance away, an end surface of the pinion 122a positioned at an axial outer side (upper side as viewed in FIG. 6). The cases 111 and 115 together combine to form a portion in which the ring gear 126 is contained, and a part of an inner surface of this portion includes a semi-circular arcuate portion that is coaxially formed with the ring gear 126 and positioned at a small distance from a tip circle of the ring gear 126. Lubricating oil is stored in the bottom portions (lower portions in FIG. 5) of the first chamber S101 and the second chamber S102. Lubricating oil stored in the second chamber S102 is lifted or circulated by means of the ring gear 126 being rotated and lubricates the transmission gear mechanism 120 from the upper side (upper side in FIG. 5). This transmission apparatus also includes two intermediate shafts between the input shaft 121 and the differential mechanism 125. The intermediate shafts are respectively provided, on output sides, with the pinion 122a and a pinion 122b, which are meshed with the ring gear 126.
According to the aforementioned transmission apparatus, in a stationary condition of the transmission apparatus, a surface of lubricating oil stored in the casing 110 is horizontal, as is clear from the dashed line b102 shown in FIG. 5. In contrast, in an operating condition of the transmission apparatus, lubricating oil stored in the casing 110 is lifted or circulated by means of the ring gear 126, which rotates in a direction of an arrow R100 shown in FIG. 5. Thus, a surface of lubricating oil stored in the second chamber S102 is inclined in such a way as to be high in a rear portion of the second chamber S102, which is positioned at the far side of the transmission gear mechanism 120, and to be low in a front portion of the second chamber S102. Therefore, lubricating oil stored in the first chamber S101 flows, through a space between the first inner surface 110a and the second inner surface 110b, into the front portion of the second chamber S102, as indicated by arrows f101 (shown in FIG. 5) and f102 (shown in FIGS. 6 and 8). Accordingly, as indicated by a solid line a 102, the surface of lubricating oil stored in the casing 110 is low in the first chamber S101, and in the front portion of the second chamber S102, and is high in a rear portion of the second chamber S102. According to the transmission apparatus with a structure as shown in FIGS. 5-8, lubricating oil flows relatively freely into the second chamber S102 from the first chamber S101, through the space between the first inner surface 110a and the second inner surface 10b. Because of the aforementioned inflow of lubricating oil, lowering of the surface of lubricating oil in the first chamber S101 is accelerated. Thus, the transmission gear mechanism 120 housed in the first chamber S101 may on occasions not sufficiently soaked or dipped in the lubricating oil, and a possibility thereby may arise of a lack of lubrication for the transmission 120. This issue can be solved by setting an increased amount of lubricating oil to be inserted into the casing 110. However, in these circumstances, a new possibility arises in so far that an energy loss (i.e., agitating loss), generated as a result of agitation or circulation of lubricating oil by means of the large diameter ring gear 126 at a time of high-speed driving, is increased. In order to reduce the degree of agitating loss, the space between the ring gear 126 and the casing 110 has conventionally been narrowed, and the part of an inner surface of the casing 110 around the ring gear 126 has been made smoother. However, unless the amount of lubricating oil inserted into the casing 110 is reduced, remarkable results cannot be expected.
Another type of transmission apparatus is known that is capable of reducing the amount of lubricating oil inserted into the casing 110, a type of transmission in which the second case 115 is provided with a case rib 116, as can be seen from a chain double-dashed line shown in FIGS. 5-8. The case rib 116 is formed so as to extend along the undersides of the pinion 122a, and of the ring gear 126, which is meshed with the pinion 122a, and so as to project from the second inner surface 110b of the second case 115 toward the first inner surface 110a. The case rib 116 includes a small circular arcuate portion 116b, which is coaxially formed with the pinion 122a and positioned therebelow, and a large circular arcuate portion 116a, which is downwardly extended from the vicinity of a front end portion of the small circular arcuate portion 116b along a circumference of the ring gear 126, and is connected to a bottom surface of the second case 115. A space e100 is defined between a tip surface of the case rib 116 and the first inner surface 110a. Within an area where the case rib 116 is provided, the respective bottom portions of the first chamber S101 and the second chamber S102 are connected to each other through the space e100. By means of the case rib 116, the bottom portions of the first chamber S101 and the second chamber S102 are divided off, except for the space e100, and in practice, there is no flow of lubricating oil indicated by the arrow f101 shown in FIG. 5. In such a condition, a flow of lubricating oil is only maintained at the upper side of the case rib 16 as indicated by arrow f102 shown in FIGS. 6 and 8, and the amount of lubricating oil that flows into the lower portion of the second chamber S102 from the lower portion of the first chamber S101 is thereby reduced. Accordingly, the degree of lowering of the surface of lubricating oil stored in the first chamber S101 resulting from the aforementioned inflow of lubricating oil is diminished, and the possibility of lack of lubrication for the transmission apparatus 120 contained in the first chamber S101 is reduced. Therefore, a pre-set value of the amount of the oil to be inserted into the casing 110 can be lowered and the agitating loss of lubricating oil caused by a rotation of the ring gear 126 at the time of high-speed driving can be thereby reduced. Moreover, a transmission apparatus provided with a case rib 116 in the casing 110 is known. However, there is no document that discloses a structure of such a transmission apparatus.
The aforementioned case rib 116 is effective to a certain degree in reducing the agitating loss of lubricating oil. However, in a case where the surface of the oil stored in the front portion of the second chamber S102 is lowered, lubricating oil stored in the first chamber S101 flows into the second chamber S102 through the space e100 between the tip surface of the case rib 116 and the first inner surface 110a, and in some cases, also through the section positioned above (upper side in FIG. 5) the case rib 116. Moreover, for purposes of avoiding interference of the case rib 116 relative to the pinion 122a and the speed-reducing large gear 126, a vertical (up and down direction in FIG. 5) position of the case rib 116 cannot be raised towards the upper side in FIG. 5. Therefore, in order to avoid loss of lubrication for the transmission apparatus 120 at a time of high-speed driving, it is necessary to set a value at a, to a degree, higher level for the amount of oil to be inserted into the casing 110. Accordingly, at a time of high-speed driving, it is difficult to avoid an increase in agitating loss caused by the rotation of the ring gear 126.
The present invention has been made in view of the above circumstances, and provides a transmission apparatus which is capable of reducing an inflow of lubricating oil from the first chamber S1 to the second chamber S2, and is capable of improving the above matters.